Declining mpg

[ddaday]

Hi all - I've seen some posts on this in the past, but not for a couple of years (and with no difinitive answers). My 2004 prius (93,000mi) used to consistently get 48-52mpg. Now it's more like 44, driving the same route, same style, etc. Worse, I sometimes only get 40 on the interstate (I never used to get less than 45). This decline has been gradual rather than sudden.

I'm good about routine maintenance. I have changed the tires, and it sounds like that might be to blame for some of the difference - but a 10-15% drop in average fuel efficiency....can that really be attributed to the tires? Funny, the Toyota dealer replaced the rear struts and the mpg seemed to decline after that. Of course, the dealership had no logical explaination for that.

Anyway, curious to see if there are thoughts about what I could look for. People think it's strange that I complain about 43-44mpg, but when you're used to 50 it's actually really disappointing. Thanks in advance...

[Matt Herring]

When did you change the tires (how many miles ago and in what month)? What tires did you replace and what are you running now? What psi are your current tires running at and what are they rated at?

All of these questions could be part of the reason for the lower mpg. New tires could put as much as a 10% hit on mpg until they are broken in. If you're running a lower psi than your old tires in comparison to their rating this could also result in lower mpg. If you changed the tires recently, lower ambient temps coupled with the new tires could certainly result in 10-15% lower mpg.

Just in temperature drop alone I am running at 61 mpg where I normally run in 63-64 mpg. Add new tires and I would not be surprised to drop under 60 mpg in current lower temps. Granted, you are in NC and you're not in that cold of temps...but the days of 80's and 90's are probably now 60's and 70's for you...or much colder on some days.

[JimboPalmer]

Any chance this is the first cold spell with your Prius?

Gas mileage changes year round, as does fuel formulations: many states go to 10% ethanol in cooler months, which typically results in 3% less MPG, although the Prius is reputed to have a higher drop. (ethanol has a higher octane rating, and absorbs water in the fuel system, but neither is a help in the Prius)

I am moving from highs near 100 degrees to highs in the 70s, so my mileage is going up (less A/C) but some parts of the country are seeing below freezing temps that will run the ICE much more, lowering MPGs.

[ken1784]

Originally Posted by ddaday

My 2004 prius (93,000mi) used to consistently get 48-52mpg.

Are you saying you were seeing such consistent mpg number all year around?
If yes, it's strange to me.

Ken@Japan

[ddaday]

Originally Posted by ken1784

Are you saying you were seeing such consistent mpg number all year around?
If yes, it's strange to me.

Ken@Japan

Thanks for all the responses. In a nutshell: No, it wasn't consistent - always declined a bit in winter and summer (with heat and ac on), but would consistently average ~49 during spring/fall and 47 during summer/winter. Lately it's been more like 44-45 consistently.

I'll check the tires. One thing I did check on my way to work was tire pressure and was surprised to see how low they were, particularly just after a tire rotation (they were down around 31). I bumped them up to 42/40 and noticed a difference on the way to work already. Perhaps this will all come down to the new tires decreasing my mpg, but hopefully I can counter some of that with extra attention to tire pressure.

[ursle]

Two things come to mind
I wonder if they tightened up your rear brakes (adjusted)
Clean your interior cabin filter(behind the glove box)
And the usuals
Did they change your oil, check the level, check your air filter,
you already found the tyre pressure low, has you gas been changed to a new ethanol ratio?

[F8L]

I think Matt covered it well. A change in tires can have a pretty large effect on MPG. Some tires are just poor performers when it comes to MPG while others need a little break in time then they get better. I saw a pretty large drop in MPG (4mpg) when I swapped from stock Integritys to the Nokian i3. I eventually gained some back but I still feel I lost maybe 2mpg. With regards to my 17" wheels with Kumho tires I noticed a permanent loss of 6mpg or so. Between the extra 7lbs per wheel/tire and the agressive tread pattern I made a poor choice with the "upgrade" if I were to look at MPG only.